C-C bond formation via C-H activation and C-N bond formation via oxidative amination catalyzed by palladium- polyoxometalate nanomaterials using dioxygen as the terminal oxidant

Article abstract of DOI:10.1002/adsc.201100362

An efficient heterogeneous palladium-polyoxometalate catalyst with the formula Pd-H₆PV₃Mo₉O₄₀/C has been successfully developed for carbon-carbon (C-C) bond formation via carbon-hydrogen (C-H) activation and car

Full text of DOI:10.1002/adsc.201100362

FULL PAPERS
DOI: 10.1002/adsc.201100362
À
À
À
C C Bond Formation via C H Activation and C N Bond
Formation via Oxidative Amination Catalyzed by Palladium-
Polyoxometalate Nanomaterials Using Dioxygen as the
Terminal Oxidant
Leng Leng Chng,a Jie Zhang,a Jinhua Yang,a Makhlouf Amoura,a
and Jackie Y. Yinga,*
a
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669
Fax : (+65)-6478-9020; e-mail: jyying@ibn.a-star.edu.sg
Received: May 8, 2011; Published online: October 20, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100362.
Abstract: An efficient heterogeneous palladium- to form the desired products. In addition, less waste
polyoxometalate catalyst with the formula Pd- is generated as no additional reagents such as organ-
H6PV3Mo9O40/C has been successfully developed for ic/inorganic oxidants are required, and water is the
À
carbon-carbon (C C) bond formation via carbon-hy- only by-product generated.
drogen (C H) activation and carbon-nitrogen (C N)
À
À
bond formation via oxidative amination using
oxygen as the terminal oxidant. The coupling pro- Keywords: C H activation; heterogeneous catalysis;
cesses are simple, and use relatively mild conditions nanomaterials; palladium; polyoxometalates
À
À
Introduction
C H activation coupling reactions, a variety of organ-
ic and inorganic oxidants such as benzoquinone,[2a,7]
À
Transition metal-catalyzed carbon-carbon (C C) PhI
A
N
bond formation via carbon-hydrogen (C H) activa- oxone,[12] has been used. These external oxidants
À
À
tion and carbon-nitrogen (C N) bond formation via would reduce the overall “greenness” of the process
À
C H activation and oxidative amination have been since more waste would be produced. A greener ap-
the subject of intense investigation in recent years.[1] proach would involve the use of O2 as the stoichio-
Various transition metals, for example, Pd,[1a–h,l,m,2] metric oxidant since water would be the only by-prod-
Ru,[1c,d,g,h,3] Rh,[1c,d,f,g,i,j,4] Cu,[1c–h,5] and Fe,[1d,h,k,6] have uct formed.[2b–k] Some of the reported C H activation
À
À
been studied in different types of C H activation and coupling reactions and oxidative amination reactions
oxidative coupling reactions. In particular, Pd-cata- used O2 as the sole oxidant,[2c,f–j] while others required
lyzed C H activation coupling reactions[2a–d,i,j] and oxi- the presence of a catalytic amount of a co-oxidant[2b,k]
À
dative amination reactions[2e–h,k] have emerged as effi- or an electron-transfer mediator (e.g., polyoxometa-
cient synthetic methods for the construction of C C lates) to facilitate the re-oxidation process.[2d,e,13] The
À
À
and C N bonds. These coupling reactions offer a use of polyoxometalates is particularly attractive as
greener and more atom-efficient approach towards they are oxidatively stable and have been used (on
the synthesis of complex molecules as they do not re-
quire the presence of a functional group such as
halide, tosylate, mesylate or organometallic moeity in
the starting materials. Hence, less waste would be
generated, and the overall cost and efficiency of the
synthetic process would be improved.
À
À
The construction of C C or C N bonds by using
only C H bonds would only occur under oxidative
conditions (Scheme 1). In homogeneous Pd-catalyzed tion or oxidative amination.
À
À
À
À
Scheme 1. C C and C N bond formation via C H activa-
2988
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2988 – 2998

À
À
À
C C Bond Formation via C H Activation and C N Bond Formation via Oxidative Amination
their own and in combination with Pd) in oxidation tained were not uniform and had a wider size distri-
reactions.[14]
bution of 3–6 nm (Figure 1, e).
The use of heterogeneous catalysts such as support-
ed Pd nanoparticles (NPs) in these C H activation gated for their catalytic activities in the oxidative
The five supported Pd nanomaterials were investi-
À
À
À
coupling reactions and oxidative amination reactions C C coupling via C H activation using the model
would represent a significant step towards green and substrates acetanilide and n-butyl acrylate (Table 1,
sustainable chemistry since these catalysts could be entries 1–5). These initial studies showed that with a
recovered and reused multiple times. Pd NPs have catalyst loading of 5 mol% of Pd and 0.4–0.6 mol% of
been developed for various catalytic reactions.[15] POM, the Pd-PV3Mo9/C nanomaterial gave the best
À
However, the use of Pd-based nanomaterials in C H result for reaction at 608C in toluene for 16 h using
activation or oxidative amination reactions is not well O2 gas as the terminal oxidant, with a 54% isolated
explored in the literature.[16] To date, the reactions are yield of the desired coupled product (Table 1,
limited to homo-coupling of 4-methylpyridine,[16a] entry 4). The coupling reaction was selective and oc-
direct acylation of aryl bromides and iodides with al- curred at the ortho position in the acetanilide sub-
dehydes,[16b] and direct arylation of heteroaromatics strate, and a minor quantity (<3%) of the disubstitut-
with aryl bromides or iodides.[16c,d] Herein, we report ed product was obtained. The use of other types of
À
our results on the efficient intermolecular C C bond POMs resulted in lower yields of the product
formation via C H activation and C N bond forma- (Table 1, entries 1–3). The presence of POM was vital
À
À
À
tion via oxidative amination catalyzed by Pd-polyoxo- for this oxidative C C coupling reaction since POM
metalate (Pd-POM) nanomaterials[17] using O2 as the served as a re-oxidation catalyst under O2. In the ab-
terminal oxidant.
sence of POM, the Pd/C nanomaterial displayed no
catalytic activity in this C H activation reaction
(Table 1, entry 5). When additional PV3Mo9 was
added to the reaction mixture, the Pd/C nanomaterial
did show some catalytic activity, but the yield of the
À
Results and Discussion
A series of supported Pd-POM nanomaterials on product was low (Table 1, entry 6). Hence, POM has
carbon (Pd-POM/C) was prepared via aqueous meth- to be incorporated during the synthesis of the Pd-
ods. The Pd NPs were produced by reduction of based nanomaterials so that they were well dispersed
Pd
ACHTUNGTRENNUNG
appropriate POM and the carbon support. The POM
was previously well dispersed onto the carbon support ed to X-ray photoelectron spectroscopy (XPS) analy-
via sonication in the solvent water (see Supporting In- sis to determine the surface oxidation state of Pd. As
formation). Using this procedure, four different types shown in Figure 2, the Pd 3d XPS spectra for the five
of Pd-POM/C nanomaterials, Pd-H3PMo12O40/C supported Pd nanomaterials could be deconvoluted
(Pd-PMo12/C), Pd-H4PVMo11O40/C (Pd-PV1Mo11/C), into two pairs of doublets. The doublet with a higher
Pd-H5PV2Mo10O40/C
(Pd-PV2Mo10/C)
and binding energy (BE) was attributed to Pd(II), while
Pd-H6PV3Mo9O40/C (Pd-PV3Mo9/C), were produced. that with a lower BE was assigned to Pd(0).[18] All
The transmission electron microscopy (TEM) images five Pd-based nanomaterials contained more oxidized
of the Pd-PMo12/C, Pd-PV1Mo11/C, Pd-PV2Mo10/C and Pd(II) surface species (~61–77%). The BEs of the
Pd-PV3Mo9/C nanomaterials showed that the Pd NPs Pd(II) 3d3/2 and 3d5/2 peaks for the four Pd-POM/C
were well dispersed with diameters of 2–3 nm nanomaterials (Pd-PMo12/C, Pd-PV1Mo11/C, Pd-
(Figure 1, a–d). The Pd loadings on these different PV2Mo10/C and Pd-PV3Mo9/C) were shifted to higher
Pd-POM/C nanomaterials were determined by induc- values (by 0.6–0.9 eV) as compared to Pd/C (see Sup-
tively-coupled plasma mass spectrometry (ICP-MS) porting Information, Table S1). In the oxidative cou-
analysis to range from 7.9 to 12.5 wt%, while the pling reaction of anilides with acrylates catalyzed by
POM loadings (14.8–15.2 wt%) showed much less var- homogeneous PdACTHUNRGTNEUNG(OAc)2, it was proposed that a
iation. The presence of both Pd and POM in these Pd(II) species was the active catalyst.[2b] In our stud-
Pd-POM/C nanomaterials was further confirmed by ies, the dramatic difference in the catalytic activities
TEM energy dispersive X-ray (EDX) analysis (Sup- between the Pd-POM/C nanomaterials and Pd/C
porting Information, Figure S1).
could be attributed to the different types of Pd(II)
The presence of POM during the synthesis of the surface species. The oxidized Pd(II) species in Pd/C
Pd NPs on the carbon support was crucial as it served might be the inactive PdO that exhibited a Pd 3d5/2
as a capping agent and ensured that the Pd NPs peak BE at 336.3 eV.[18] For the four Pd-POM/C nano-
achieved were small, uniform and well-dispersed. materials, the oxidizing POMs that were in close con-
When the Pd NPs were synthesized in the absence of tact with the Pd NPs could have selectively modified
the POM (i.e., as Pd/C nanomaterial), the Pd NPs ob- the latterꢁs surface to produce catalytically active
Pd(II) species with higher BEs.
Adv. Synth. Catal. 2011, 353, 2988 – 2998
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2989

Leng Leng Chng et al.
FULL PAPERS
Figure 1. TEM of (a) Pd-PMo12/C, (b) Pd-PV1Mo11/C, (c) Pd-PV2Mo10/C, (d) Pd-PV3Mo9/C, and (e) Pd/C.
Using the most active Pd-PV3Mo9/C catalyst, the dichloroethane resulted in lower isolated yields of the
À
reaction conditions for the oxidative C C coupling re- product (Table 2, entries 3–5), while no product was
action was further optimized with respect to the sol- formed when N,N-dimethylformamide (DMF) was
vent, temperature and the amount of reactant used used as the solvent (Table 2, entry 2). Since O2 was
(Table 2). The nature of the reaction media was first used as the terminal oxidant, its presence was essen-
À
investigated with polar and non-polar solvents. The tial in this C H activation reaction as evident from
non-polar solvent toluene was found to be the most the lack of catalytic activity when the reaction was
effective for this coupling reaction (Table 2, entry 1). performed under argon (Table 2, entry 6), whereas in
Polar solvents such as 1,4-dioxane, methanol and 1,2- the presence of air, a lower yield of the product was
2990
asc.wiley-vch.de
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2988 – 2998

À
À
À
C C Bond Formation via C H Activation and C N Bond Formation via Oxidative Amination
[a]
À
À
Table 1. C C bond formation via C H activation catalyzed by palladium-based nanomaterials.
Entry
Catalyst
Pd loading [wt%][b]
POM loading [wt%][b]
POM used [mol%]
Yield [%][c]
1
2
3
4
Pd-PMo12/C
Pd-PV1Mo11/C
Pd-PV2Mo10/C
Pd-PV3Mo9/C
Pd/C
8.1
15.2
15.1
14.8
15.2
0.0
0.5
0.4
0.4
0.6
0.0
0.6
44
39
41
54
0
12.5
10.4
7.9
17.0
17.0
5
6[d]
Pd/C+PV3Mo9
0.0[e]
9
[a]
Reaction conditions: Pd catalyst (5 mol%), 1a (0.2 mmol) and 2a (0.4 mmol), toluene (0.45 mL, purged with O2 for
5 min), 608C, under O2.
Pd or POM content in nanomaterials was determined from ICP-MS analysis.
Isolated and unoptimized yield based on 1a.
0.6 mol% of PV3Mo9 (H6PMo9V3O40) was added to the reaction mixture.
POM loading in Pd/C.
[b]
[c]
[d]
[e]
Figure 2. Pd 3d XPS spectra of (a) Pd-PMo12/C, (b) Pd-PV1Mo11/C, (c) Pd-PV2Mo10/C, (d) Pd-PV3Mo9/C and (e) Pd/C. Ap-
proximately 77%, 71%, 72%, 61% and 68% of the Pd in the samples (a), (b), (c), (d) and (e) are in the Pd(II) oxidation
state, respectively.
attained (Table 2, entry 7). Increasing the Pd catalyst yield of the product was increased by 22% to 76%.
loading to 10 mol% led to a small increase in the At 808C, the yield of the product was reduced to 65%
product yield (Table 2, entry 12) at 608C. In contrast, and 60% upon decreasing the Pd catalyst loading to
the catalytic activity of the Pd-PV3Mo9/C nanomateri- 3 mol% (Table 2, entry 10) and reducing the amount
al could be improved substantially by increasing the of butyl acrylate to 1.2 molar equivalents (Table 2,
reaction temperature (Table 2, entries 8 and 9). Upon entry 11), respectively.
heating the reaction mixture at 808C for 16 h, the
Adv. Synth. Catal. 2011, 353, 2988 – 2998
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2991

Leng Leng Chng et al.
FULL PAPERS
[a]
À
À
Table 2. C C bond formation via C H activation catalyzed by Pd-PV3Mo9/C nanomaterial.
Entry
Pd [mol%]
Butyl acrylate [mmol]
Solvent
Temperature [8C]
Atmosphere
Yield [%][b]
1
2
3
4
5
5
5
5
5
5
5
5
5
3
5
10
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.24
0.4
toluene
DMF
1,4-dioxane
CH3OH
1,2-dichloroethane
toluene
toluene
toluene
toluene
toluene
60
60
60
60
60
60
60
70
80
80
80
60
O2
O2
O2
O2
O2
argon
air
O2
O2
O2
O2
54
0
40
13
48
0
37
62
76
65
60
58
5
6[c]
7[d]
8
9
10[e]
11
12[f]
toluene
toluene
O2
[a]
Reaction conditions: Pd catalyst, PV3Mo9 (0.6 mol%), 1a (0.2 mmol), 2a (0.24 or 0.4 mmol), solvent (0.45 mL, purged
with O2 for 5 min), 60–808C, under O2.
Isolated yield based on 1a.
Toluene used was stored in the argon glovebox.
Toluene was purged with air for 5 min.
[b]
[c]
[d]
[e]
[f]
0.36 mol% of PV3Mo9 was used.
1.2 mol% of PV3Mo9 was used.
The recyclability of the Pd-PV3Mo9/C catalyst was loading back to its original value) was necessary as
investigated by recovering the nanomaterial via cen- the yield of the product would decrease substantially
trifugation, and reloading the recovered solids with (by 23%) if such a step was not undertaken. There
PV3Mo9 prior to the next run (Supporting Informa- was no change in the size of the Pd NPs after the cou-
tion, Table S2). ICP-MS analysis of the PV3Mo9 load- pling reaction, as confirmed by their TEM images
ing of the recovered catalyst showed that there was a (Figure 3). The Pd-PV3Mo9/C catalyst retained 97%
significant loss of the PV3Mo9 content (54% reduc- of its reactivity (74% yield) after the first recycle, and
tion) after the coupling reaction. Reloading the recov- 79% of its activity (60% yield) was still preserved in
ered catalyst with PV3Mo9 (to restore the PV3Mo9 the fourth run. To determine if the Pd-PV3Mo9/C cat-
À
Figure 3. TEM images of Pd-PV3Mo9/C (a) before and (b) after the C C bond formation reaction.
2992
asc.wiley-vch.de
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2988 – 2998

À
À
À
C C Bond Formation via C H Activation and C N Bond Formation via Oxidative Amination
À
À
Table 3. Scope of C C bond formation via C H activation
alyst was leaching to form a homogeneous active spe-
cies, a reaction was prematurely stopped after heating
at 808C under O2 for 4 h (i.e., 40% product yield by
GC-MS analysis), and filtered hot to remove the
solids. Heating of the filtrate at 808C under O2 for
12 h showed no further product formation. ICP-MS
analysis of the filtrate confirmed that only traces of
Pd were detected in the solution (7 ppm).
catalyzed by Pd-PV3Mo9/C nanomaterial.[a]
The surface chemistry of the Pd-PV3Mo9/C catalyst
À
before and after the C C coupling reaction (first and
fourth runs) was subjected to XPS analysis (Support-
ing Information, Figure S2). There was no significant
change in the quantity of the oxidized active Pd(II)
species in the Pd-PV3Mo9/C catalyst before and after
À
the C C coupling reactions. Hence, the small de-
crease in the isolated yield of the C C coupled prod-
À
uct in the recycling experiments was probably not due
to deactivation of the active catalytic species, but
could be attributed to some loss of the nanomaterial
in the recovery process.
Using the optimized set of reaction conditions, we
À
investigated the scope of the oxidative C C coupling
reactions with a variety of substituted anilides and
acrylates (Table 3). Functional groups on the acetani-
lide substrates have a significant impact on the effica-
cy of the coupling reaction. With an electron-donating
CH3 substituent at the para position to the acetamide
moiety, the coupling reaction proceeded smoothly
with a good yield of 79% (Table 3, 3b). On the other
hand, the presence of a stronger electron-donating
group such as OCH3, or an electron-withdrawing
group such as F or CF3 at the para position to the
acetamide moiety resulted in lower yields of the de-
sired products (46–63%, Table 3, 3c–e). Electron-do-
nating and electron-withdrawing functional groups
such as CH3, OCH3, F, Cl and CF3 at the meta posi-
tion to the acetamide moiety were also examined.
Good yields of 78–80% were attained with the elec-
tron-donating CH3 or the electron-withdrawing F
group at the meta position to the acetamide moiety
(Table 3, 3f and 3h). However, low to moderate yields
of 21–53% were achieved in the presence of a stron-
ger electron-donating group such as OCH3 or elec-
tron-withdrawing groups such as Cl and CF3 at the
meta position (Table 3, 3g, 3i, 3j).
[a]
Reaction conditions: Pd catalyst (5 mol%), PV3Mo9
(0.6 mol%), 1 (0.2 mmol) and 2 (0.4 mmol), toluene
(0.45 mL, purged with O2 for 5 min), 808C, 16 h, under
O2.
From the above results, it appeared that the oxida-
À
tive C C coupling reaction catalyzed by Pd-PV3Mo9/
C catalyst was suitable for anilide substrates with
weakly activating functional groups (CH3). Anilide
substrates with electron-withdrawing groups (F, Cl
and CF3) or stronger electron-donating groups
(OCH3) would lead to lower product yields. However,
the substrate 1h (product 3h, Table 3) did not follow pairs of electrons could result in an electron-releasing
the above trend. In this case, the F substituent was at influence through conjugation. Hence, the combina-
2
À
the para position to the Csp H bond undergoing the tion of these two factors could result in an overall
oxidative coupling reaction. Although the F group weakly activating effect, leading to a good yield of
has an inductive electron-withdrawing effect, its lone- the desired product.
Adv. Synth. Catal. 2011, 353, 2988 – 2998
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2993

Leng Leng Chng et al.
FULL PAPERS
[a]
À
Table 4. C N bond formation via oxidative amination catalyzed by palladium-based nanomaterials.
Entry
Catalyst
Pd loading [wt%][b]
POM loading [wt%][b]
POM used [mol%]
Yield [%][c]
1
2
3
4
Pd-PMo12/C
Pd-PV1Mo11/C
Pd-PV2Mo10/C
Pd-PV3Mo9/C
Pd/C
8.1
15.2
15.1
14.8
15.2
0.0
0.5
0.4
0.4
0.6
0.0
0.6
86
78
74
87
0
12.5
10.4
7.9
17.0
17.0
5
6[d]
Pd/C+PV3Mo9
0.0[e]
<1
[a]
Reaction conditions: Pd catalyst (5 mol%), 4a (0.2 mmol) and 2a (0.6 mmol), DMF (0.4 mL, purged with O2 for 5 min),
608C, under O2.
The Pd and POM contents in nanomaterials were determined from ICP-MS analysis.
Isolated and unoptimized yield based on 4a.
0.6 mol% PV3Mo9 (H6PMo9V3O40) was added to the reaction mixture.
POM loading in Pd/C.
[b]
[c]
[d]
[e]
The effect of different substituents on the amide
To further optimize the oxidative amination reac-
moiety was compared using the acetyl, pivaloyl and tion conditions over the Pd-PV3Mo9/C catalyst, the re-
benzoyl substitution. Replacing the acetyl group with action media, temperature and time were examined
the pivaloyl group afforded comparable yields of the systematically (Table 5). The solvent used has a signif-
desired coupled product (Table 3, 3a and 3k). Howev- icant impact on this oxidative amination reaction.
er, replacement with the benzoyl group resulted in a DMF was the best solvent for this reaction (Table 5,
lower product yield of 45% (Table 3, 3l). Thus, ali- entry 1). No product was obtained when other polar
À
phatic acyl groups were more tolerated in this C C solvents (i.e., 1,4-dioxane, methanol and 1,2-dichloro-
coupling reaction as compared to the aromatic acyl ethane) and non-polar solvent (i.e., toluene) were
group.
used (Table 5, entries 2–5). O2 gas was again critical
Different types of acrylates were also investigated in this oxidative amination reaction. No product was
with the Pd-PV3Mo9/C nanomaterial. In addition to n- formed when the reaction was performed under argon
butyl acrylate, other acrylates such as methyl acrylate, (Table 5, entry 6), and a lower yield of 63% was ach-
ethyl acrylate and isobutyl acrylates also coupled ef- ieved when air was used instead of O2 (Table 5,
fectively in this reaction (Table 3, 3m–o). However, if entry 7). The catalytic activity of the Pd-PV3Mo9/C
the acrylate substrate became too bulky (e.g., with a nanomaterial was not improved through the use of a
t-Bu group), no coupling reaction occurred.
higher reaction temperature (Table 5, entry 8) or a
À
Besides the oxidative C C coupling reactions, the longer reaction time (Table 5, entry 10). In fact, with
Pd-POM/C nanomaterials were also examined for the a higher reaction temperature of 808C, the yield of
À
C N bond formation via oxidative amination using the product was reduced to 66%. Decreasing the
the model substrates diphenylamine and n-butyl acry- amount of n-butyl acrylate to 1.5 molar equivalent
late (Table 4, entries 1–5). The initial screening results (Table 5, entry 9) resulted in a lower product yield at
showed that with 5 mol% Pd and a reaction time of 608C.
6 h at 608C in DMF under O2, the Pd-PMo12/C and
The recyclability of the Pd-PV3Mo9/C catalyst was
Pd-PV3Mo9/C nanomaterials displayed the highest ac- examined after recovery of the nanomaterial via cen-
tivities, giving the coupled amine product in 86–87% trifugation, followed by reloading of the catalyst with
isolated yields (Table 4, entries 1 and 4). The other PV3Mo9 (Supporting Information, Table S3). Reload-
two Pd-POM/C nanomaterials gave slightly lower ing of the catalyst with PV3Mo9 was essential as the
yields of the desired product (Table 4, entries 2 and activity of the catalyst would decrease dramatically
À
3). As with the oxidative C C coupling reaction, the (by 87% to 10% yield) without reloading PV3Mo9.
Pd/C nanomaterial displayed no catalytic activity in ICP-MS analysis of the PV3Mo9 loading in the recov-
this oxidative amination reaction (Table 4, entry 5), ered catalyst (prior to reloading PV3Mo9) showed that
even when additional PV3Mo9 was added to the reac- there was a 70% loss of the PV3Mo9 content after the
tion mixture (Table 4, entry 6).
coupling reaction. TEM analysis of the recovered cat-
alyst confirmed that there were no significant changes
2994
asc.wiley-vch.de
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2988 – 2998

À
À
À
C C Bond Formation via C H Activation and C N Bond Formation via Oxidative Amination
[a]
À
Table 5. C N bond formation via oxidative amination catalyzed by Pd-PV3Mo9/C nanomaterial.
Entry
Butyl acrylate [mmol]
Solvent
Temperature [8C]
Atmosphere
Time [h]
Yield [%][b]
1
2
3
4
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.3
0.6
DMF
60
60
60
60
60
60
60
80
60
60
O2
O2
O2
O2
O2
argon
air
O2
O2
6
6
6
6
6
6
6
6
6
87
toluene
1,4-dioxane
CH3OH
1,2-dichloroethane
DMF
DMF
DMF
DMF
DMF
<1
<1
<1
<1
0
63
66
66
82
5
6[c]
7[d]
8
9
10
O2
14
[a]
Reaction conditions: Pd catalyst (5 mol%), PV3Mo9 (0.6 mol%), 4a (0.2 mmol), 2a (0.3 or 0.6 mmol), solvent (0.4 mL,
purged with O2 for 5 min), 60–808C, under O2.
Isolated yield based on 4a.
DMF used was stored in the argon glovebox.
DMF was purged with air for 5 min.
[b]
[c]
[d]
to the size and morphology of the Pd NPs after the filtrate was heated further at 608C under O2 for 5.5 h.
coupling reaction (Figure 4). XPS analysis of the Pd- There was an increase in the product yield (from 30%
À
PV3Mo9/C catalyst after the C N coupling reaction to 45%), indicating that some homogeneous Pd spe-
(fourth run) revealed that a comparable amount of cies were leached into the reaction mixture, giving
the oxidized active Pd(II) species was still present in rise to the catalytic activity in the heated filtrate.
the nanomaterial (Supporting Information, Fig-
To investigate the scope of the oxidative amination
ure S3). After one successive cycle, the catalyst re- reaction catalyzed by the Pd-PV3Mo9/C catalyst, dif-
tained 91% of its activity (79% yield), and after three ferent types of secondary amines and olefins were ex-
successive cycles, 77% of the reactivity (67% yield) amined (Table 6). Substituents on the aromatic
was obtained. To verify if a homogeneous active spe- amines have a considerable effect on the amination
cies was leached from the Pd-PV3Mo9/C catalyst, a re- reaction. With a CH3 substituent at the para position
action mixture was heated at 608C under O2 for on one of the aromatic rings, the electron-donating
30 min (30% product yield by GCMS analysis), and CH3 group resulted in a lower yield of the coupled
then filtered hot to remove the catalyst. The resulting amine product (77%, Table 6, 5b). This yield was fur-
À
Figure 4. TEM images of Pd-PV3Mo9/C (a) before and (b) after the C N bond formation reaction.
Adv. Synth. Catal. 2011, 353, 2988 – 2998
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2995

Leng Leng Chng et al.
FULL PAPERS
À
Table 6. Scope of C N bond formation via oxidative amina-
at the meta position resulted in lower yields of the de-
sired products (12–57%, Table 6, 5g–5i). Replacement
of an aromatic ring with an alkyl chain in the secon-
dary amine substrate would also result in a low yield
of the coupled amine product (Table 6, 5j).
tion catalyzed by Pd-PV3Mo9/C nanomaterial.[a]
The above results showed that, in general, the oxi-
dative amination reaction catalyzed by Pd-PV3Mo9/C
catalyst was more suited for secondary amine sub-
strates with weakly activating functional groups (CH3
or F) in the aromatic ring. Secondary amine sub-
strates with electron-withdrawing groups (Cl and Br)
or stronger electron-donating groups (OCH3) would
result in lower product yields. For the substrate 4d
(product 5d, Table 6), the lower yield could be due to
the presence of the para CH3 group in both of the ar-
omatic rings.
Various acrylates were tested with the Pd-PV3Mo9/
C catalyst. In general, the acrylates (methyl, ethyl,
isobutyl and tert-butyl acrylates) coupled smoothly in
this reaction to give the desired product in good
yields (Table 6, 5k–5n). The electron-deficient olefin,
methyl vinyl ketone, also reacted with diphenylamine,
although the yield of the product was low (Table 6,
5o).
Conclusions
In conclusion, we have successfully developed an effi-
À
cient heterogeneous Pd-PV3Mo9/C catalyst for the C
À
À
C bond formation via C H activation and the C N
bond formation via oxidative amination using O2 as
the terminal oxidant under relatively mild conditions.
The coupling reactions are simple, green processes as
no additional reagents (such as acid, base or organic/
inorganic oxidant) besides the reactants are required,
and water is the only by-product that is generated.
[a]
Reaction conditions: Pd catalyst (5 mol%), PV3Mo9
(0.6 mol%),
4 (0.2 mmol) and 2 (0.6 mmol), DMF
(0.4 mL, purged with O2 for 5 min), 608C, 6 h, under O2.
20 h at 808C.
24 h at 608C.
Experimental Section
[b]
[c]
À
General Procedure for the C C Coupling Reaction
of Anilides with Acrylates (Table 3, 3a–o)
ther decreased to 57% when both aromatic rings con- A mixture of the Pd-PV3Mo9/C nanomaterial (5 mol% Pd,
0.6 mol% PV3Mo9), anilide (0.2 mmol), acrylate (0.4 mmol)
and dry toluene (0.45 mL, purged with O2 for 5 min) were
added to an 8-mL glass vial equipped with a screw cap. The
headspace in the glass vial was purged with O2. The reaction
mixture was heated at 808C under O2 for 16 h. The mixture
was centrifuged at 7500 rpm for 10 min. The organic layer
containing the product was separated, and the solids were
washed with hexane/dichloromethane (2ꢂ3 mL). The sol-
vent was removed by rotary evaporation, followed by purifi-
cation using column chromatography (silica gel, 10%
tained a CH3 group at their para positions (Table 6,
5d). When the CH3 substituent was at the meta posi-
tion on either one or both aromatic rings, the cou-
pling reaction went efficiently and good yields of 86–
90% were achieved (Table 6, 5c and 5e). A good yield
of 82% was also achieved when the electron-with-
drawing F group was present at the para position on
one of the aromatic rings (Table 6, 5f). However, the
presence of
a stronger electron-donating group
(OCH3) or an electron-withdrawing group (Cl or Br) EtOAc in dichloromethane).
2996
asc.wiley-vch.de
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2988 – 2998

À
À
À
C C Bond Formation via C H Activation and C N Bond Formation via Oxidative Amination
À
f) S. R. Fix, J. L. Brice, S. S. Stahl, Angew. Chem. 2002,
General Procedure for the C N Coupling Reaction
of Secondary Amines with Olefins (Table 6, 5a–o)
114, 172–174; Angew. Chem. Int. Ed. 2002, 41, 164–166;
g) J. L. Brice, J. E. Harang, V. I. Timokhin, N. R. Anas-
tasi, S. S. Stahl, J. Am. Chem. Soc. 2005, 127, 2868–
2869; h) M. M. Rogers, V. Kotov, J. Chatwichien, S. S.
Stahl, Org. Lett. 2007, 9, 4331–4334; i) Y.-H. Zhang, B.-
F. Shi, J.-Q. Yu, J. Am. Chem. Soc. 2009, 131, 5072–
5074; j) K. M. Engle, D.-H. Wang, J.-Q. Yu, J. Am.
Chem. Soc. 2010, 132, 14137–14151; k) V. I. Timokhin,
S. S. Stahl, J. Am. Chem. Soc. 2005, 127, 17888–17893.
[3] a) G. Deng, L. Zhao, C.-J. Li, Angew. Chem. 2008, 120,
6374–6378; Angew. Chem. Int. Ed. 2008, 47, 6278–6282;
b) L. Ackermann, P. Novꢃk, R. Vicente, N. Hofmann,
Angew. Chem. 2009, 121, 6161–6164; Angew. Chem.
Int. Ed. 2009, 48, 6045–6048; c) S. Inoue, H. Shiota, Y.
Fukumoto, N. Chatani, J. Am. Chem. Soc. 2009, 131,
6898–6899; d) N. M. Neisius, B. Plietker, Angew. Chem.
2009, 121, 5863–5866; Angew. Chem. Int. Ed. 2009, 48,
5752–5755.
A mixture of the Pd-PV3Mo9/C nanomaterial (5 mol% Pd,
0.6 mol% PV3Mo9), secondary amine (0.2 mmol), olefin
(0.6 mmol) and dry DMF (0.4 mL, purged with O2 for
5 min) were added to an 8-mL glass vial equipped with a
screw cap. The headspace in the glass vial was purged with
O2. The reaction mixture was heated at 608C under O2 for
6 h. The mixture was centrifuged at 7500 rpm for 10 min.
The organic layer containing the product was separated, and
the solids were washed with hexane/dichloromethane (2ꢂ
3 mL). The solvent was removed by rotary evaporation, di-
chloromethane (20 mL) was added, and the organic layer
was washed with water (3ꢂ10 mL). The organic layer was
separated, dried over Na2SO4, and the solvent was removed
by rotary evaporation, followed by purification using
column chromatography (silica gel, dichloromethane/hex-
anes=1:1).
[4] a) M. Kim, J. Kwak, S. Chang, Angew. Chem. 2009,
121, 9097–9101; Angew. Chem. Int. Ed. 2009, 48, 8935–
8939; b) H.-Y. Thu, G. S.-M. Tong, J.-S. Huang, S. L.-F.
Chan, Q.-H. Deng, C.-M. Che, Angew. Chem. 2008,
120, 9893–9897; Angew. Chem. Int. Ed. 2008, 47, 9747–
9751; c) Z. Ye, W. Wang, F. Luo, S. Zhang, J. Cheng,
Org. Lett. 2009, 11, 3974–3977; d) D. E. Olson, J. Du
Bois, J. Am. Chem. Soc. 2008, 130, 11248–11249.
Acknowledgements
This work is funded by the Institute of Bioengineering and
Nanotechnology (Biomedical Research Council, Agency for
Science, Technology and Research, Singapore).
[5] a) R. J. Phipps, M. J. Gaunt, Science 2009, 323, 1593–
1597; b) F. Besseliꢄvre, S. Piguel, Angew. Chem. 2009,
121, 9717–9720; Angew. Chem. Int. Ed. 2009, 48, 9553–
9556; c) Z. Li, C.-J. Li, J. Am. Chem. Soc. 2004, 126,
11810–11811; d) Y. Wei, H. Zhao, J. Kan, W. Su, M.
Hong, J. Am. Chem. Soc. 2010, 132, 2522–2523.
[6] a) Z. Li, L. Cao, C.-J. Li, Angew. Chem. 2007, 119,
6625–6627; Angew. Chem. Int. Ed. 2007, 46, 6505–6507;
b) Z. Li, R. Yu, H. Li, Angew. Chem. 2008, 120, 7607–
7610; Angew. Chem. Int. Ed. 2008, 47, 7497–7500;
c) Y.-Z. Li, B.-J. Li, X.-Y. Lu, S. Lin, Z.-J. Shi, Angew.
Chem. 2009, 121, 3875–3878; Angew. Chem. Int. Ed.
2009, 48, 3817–3820; d) J. Norinder, A. Matsumoto, N.
Yoshikai, E. Nakamura, J. Am. Chem. Soc. 2008, 130,
5858–5859.
[7] a) D. J. Covell, M. C. White, Angew. Chem. 2008, 120,
6548–6551; Angew. Chem. Int. Ed. 2008, 47, 6448–6451;
b) S. A. Reed, A. R. Mazzotti, M. C. White, J. Am.
Chem. Soc. 2009, 131, 11701–11706; c) R. Giri, J. K.
Lam, J.-Q. Yu, J. Am. Chem. Soc. 2010, 132, 686–693.
[8] a) A. R. Dick, K. L. Hull, M. S. Sanford, J. Am. Chem.
Soc. 2004, 126, 2300–2301; b) L. V. Desai, K. L. Stow-
ers, M. S. Sanford, J. Am. Chem. Soc. 2008, 130, 13285–
13293.
[9] a) M. Wasa, K. M. Engle, J.-Q. Yu, J. Am. Chem. Soc.
2010, 132, 3680–3681; b) Y. Lu, D.-H. Wang, K. M.
Engle, J.-Q. Yu, J. Am. Chem. Soc. 2010, 132, 5916–
5921; c) D. R. Stuart, E. Villemure, K. Fagnou, J. Am.
Chem. Soc. 2007, 129, 12072–12073.
References
[1] Reviews: a) T. W. Lyons, M. S. Sanford, Chem. Rev.
2010, 110, 1147–1169; b) X. Chen, K. M. Engle, D.-H.
Wang, J.-Q. Yu, Angew. Chem. 2009, 121, 5196–5217;
Angew. Chem. Int. Ed. 2009, 48, 5094–5115; c) R. Giri,
B.-F. Shi, K. M. Engle, N. Maugel, J.-Q. Yu, Chem. Soc.
Rev. 2009, 38, 3242–3272; d) L. Ackermann, R. Vicente,
A. R. Kapdi, Angew. Chem. 2009, 121, 9976–10011;
Angew. Chem. Int. Ed. 2009, 48, 9792–9826; e) O. Dau-
gulis, H.-Q. Do, D. Shabashov, Acc. Chem. Res. 2009,
42, 1074–1086; f) F. Collet, R. H. Dodd, P. Dauban,
Chem. Commun. 2009, 5061–5074; g) G. Dyker, Angew.
Chem. 1999, 111, 1808–1822; Angew. Chem. Int. Ed.
1999, 38, 1698–1712; h) C.-J. Li, Acc. Chem. Res. 2009,
42, 335–344; i) J. C. Lewis, R. G. Bergman, J. A.
Ellman, Acc. Chem. Res. 2008, 41, 1013–1025; j) D. A.
Colby, R. G. Bergman, J. A. Ellman, Chem. Rev. 2010,
110, 624–655; k) A. A. O. Sarhan, C. Bolm, Chem. Soc.
Rev. 2009, 38, 2730–2744; l) S. S. Stahl, Angew. Chem.
2004, 116, 3480–3501; Angew. Chem. Int. Ed. 2004, 43,
3400–3420; m) R. I. McDonald, G. Liu, S. S. Stahl,
Chem. Rev. 2011, 111, 2981–3019.
[2] a) M. D. K. Boele, G. P. F. van Strijdonck, A. H. M.
de Vries, P. C. J. Kamer, J. G. de Vries, P. W. N. M. van
Leeuwen, J. Am. Chem. Soc. 2002, 124, 1586–1587;
b) J.-R. Wang, C.-T. Yang, L. Liu, Q.-X. Guo, Tetrahe-
dron Lett. 2007, 48, 5449–5453; c) M. Dams, D. E.
De Vos, S. Celen, P. A. Jacobs, Angew. Chem. 2003,
115, 3636–3639; Angew. Chem. Int. Ed. 2003, 42, 3512–
3515; d) T. Yokota, M. Tani, S. Sakaguchi, Y. Ishii, J.
Am. Chem. Soc. 2003, 125, 1476–1477; e) Y. Obora, Y.
Shimizu, Y. Ishii, Org. Lett. 2009, 11, 5058–5061;
[10] a) R. Giri, J.-Q. Yu, J. Am. Chem. Soc. 2008, 130,
14082–14083; b) K. L. Hull, M. S. Sanford, J. Am.
Chem. Soc. 2007, 129, 11904–11905.
[11] a) X. Wang, L. Truesdale, J.-Q. Yu, J. Am. Chem. Soc.
2010, 132, 3648–3649; b) N. P. Grimster, C. Gauntlett,
C. R. A. Godfrey, M. J. Gaunt, Angew. Chem. 2005,
Adv. Synth. Catal. 2011, 353, 2988 – 2998
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2997

Leng Leng Chng et al.
FULL PAPERS
117, 3185–3189; Angew. Chem. Int. Ed. 2005, 44, 3125–
3129; c) D. R. Stuart, K. Fagnou, Science 2007, 316,
1172–1175.
Teuma, M. Gꢅmez, Eur. J. Inorg. Chem. 2008, 3577–
3586; f) C. P. Mehnert, J. Y. Ying, Chem. Commun.
1997, 2215–2216; g) C. P. Mehnert, D. W. Weaver, J. Y.
Ying, J. Am. Chem. Soc. 1998, 120, 12289–12296.
[16] a) L. M. Neal, H. E. Hagelin-Weaver, J. Mol. Catal. A:
Chem. 2008, 284, 141–148; b) L. Adak, S. Bhadra, B. C.
Ranu, Tetrahedron Lett. 2010, 51, 3811–3814; c) D.
Saha, L. Adak, B. C. Ranu, Tetrahedron Lett. 2010, 51,
5624–5627; d) V. A. Zinovyeva, M. A. Vorotyntsev, I.
Bezverkhyy, D. Chaumont, J.-C. Hierso, Adv. Funct.
Mater. 2011, 21, 1064–1075.
[12] K. L. Hull, E. L. Lanni, M. S. Sanford, J. Am. Chem.
Soc. 2006, 128, 14047–14049.
[13] T. A. Dwight, N. R. Rue, D. Charyk, R. Josselyn, B.
DeBoef, Org. Lett. 2007, 9, 3137–3139.
[14] a) V. Maxym, S.-R. Dorit, H. Adina, M. Galia, N.
Ronny, Top. Catal. 2005, 34, 93–99; b) E. Monflier, E.
Blouet, Y. Barbaux, A. Mortreux, Angew. Chem. 1994,
106, 2183–2185; Angew. Chem. Int. Ed. Engl. 1994, 33,
2100–2102; c) Y. Obora, Y. Okabe, Y. Ishii, Org.
Biomol. Chem. 2010, 8, 4071–4073.
[15] a) J. M. Campelo, D. Luna, R. Luque, J. M. Marinas,
A. A. Romero, ChemSusChem 2009, 2, 18–45; b) L.
Djakovitch, K. Kçhler, J. G. de Vries, Nanoparticles
and Catalysis, (Ed.: D. Astruc), Wiley, 2008, Chap. 10,
pp 303–348; c) N. Erathodiyil, S. Ooi, A. M. Seayad, Y.
Han, S. S. Lee, J. Y. Ying, Chem. Eur. J. 2008, 14, 3118–
3125; d) K. Sawai, R. Tatumi, T. Nakahodo, H. Fuji-
hara, Angew. Chem. 2008, 120, 7023–7025; Angew.
Chem. Int. Ed. 2008, 47, 6917–6919; e) J. Durand, E.
[17] For a review on the synthesis of metal nanostructures
using polyoxometalates, see B. Keita, T. Liu, L. Nadjo,
J. Mater. Chem. 2009, 19, 19–33.
[18] a) C. D. Wagner, A. V. Naumkin, A. Kraut-Vass, J. W.
Allison, C. J. Powell, J. R. Rumble Jr, NIST Standard
Reference Database 20, Version 3.2 (Web Version);
b) J. F. Moulder, W. F. Stickle, P. E. Sobol, K. D.
Bomben, in: Handbook of X-ray Photoelectron Spec-
troscopy, (Ed.: J. Chastain), Physical Electronics Divi-
sion, Perkin–Elmer Corporation, Eden Prairie, MN,
1992.
2998
asc.wiley-vch.de
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2988 – 2998